The present invention relates to a process and to a reactor for the manufacture of norbornene (1,2,2-bicyclo[2.2.1]hept-2-ene) from dicyclopentadiene (DCPD) and ethylene.
The synthesis of norbornene was described for the first time in 1941 by L. M. Joshel and L. W. Butz (J. Am. Chem. Soc., 63, 3350).
U.S. Pat. No. 2,340,908 discloses the reaction of DCPD and ethylene at a temperature of approximately 200xc2x0 C. under a pressure of 50 to approximately 100 bar.
U.S. Pat. No. 3,007,977 discloses the synthesis of norbornene from a mixture of cyclopentadiene (CPD) and DCPD, the use of the mixture being regarded as favouring the control of the reaction conditions.
U.S. Pat. No. 3,763,253 discloses a highly selective process for the manufacture of norbornenes which consists of:
(1) introducing a mixture of a lower olefin and of DCPD having a temperature of greater than approximately 190xc2x0 C., in particular of approximately 200-325xc2x0 C., into a reactor with a substantial excess of olefin, for example at an olefin/DCPD molar ratio of 1:0.5 to 40:1,
(2) maintaining the conditions of temperature, of pressure and of residence time in the reactor so that the reactants and the products remain in the vapour phase and so that the formation of the norbornenes is favoured, in particular at a temperature of 200-325xc2x0 C., a pressure of 6.8 to 136 Pa (100 to 2000 psi) and a residence time of 0.5 to 20 minutes, and
(3) recovering the norbornenes.
Patents DD-140,874 and DE-203,313 disclose a continuous process for the manufacture of norbornene by reaction of DCPD and/or CPD and ethylene under reaction conditions under which not only the reactants but also the desired norbornene are obtained in the gas phase, according to which process 1 mol of DCPD is mixed with 2-50 mol of ethylene and the reaction is carried out at 250-340xc2x0 C. under a pressure of 2-20 MPa, excess ethylene being conveyed, before the reaction region, under the reaction pressure at a temperature of less than 190xc2x0 C., through the liquid DCPD and then reacting to give the norbornene in the reaction region, the norbornene being withdrawn in the gaseous form at the top of the reaction region; according to DD-140,874, liquid compounds with high boiling points are withdrawn below the reaction region at temperatures of 150-220xc2x0 C. in an amount of less than 3% by weight with respect to the DCPD introduced into the reaction system, their composition being regulated so as to comprise less than 1% by weight of norbornene; according to DE-203,313, liquid compounds with high boiling points are withdrawn from the region for mixing the DCPD with the ethylene, the liquid DCPD having a mean residence time in the mixing region of less than 60 minutes.
German Patent DD-144, 257 relates to a reactor for the synthesis of norbornene from DCPD and ethylene, the reaction space being divided by two concentrically positioned pipes into an annular space, in which cleavage of the DCPD predominantly takes place, and into an internal space, in which synthesis of the norbornene predominantly takes place.
German Patent DD-215, 078 relates to a process for the continuous manufacture of pure norbornene under reaction conditions according to which not only the reactants CPD and ethylene but also the norbornene are maintained in the gas phase; 1 mol of CPD is reacted with 1 to 25 mol of ethylene at 523-613xc2x0 K. and under a pressure of 2-20 MPa; a DCPD concentrate, which comprises up to 20% of codimers of CPD with methylcyclopentadiene, piperylene, isoprene and butadiene, is mixed with the ethylene in a mixing region at 433-473xc2x0 K. with a residence time of 10 to 30 minutes; the compounds with high boiling points formed in the mixing region are withdrawn at the lower end; the conversion to norbornene takes place in a reaction region following the mixing region and the reaction products are withdrawn at the top of the reaction region; the reaction products are separated in a two-stage distillation region which follows, so that methylnorbornene, methyltetrahydroindenes, DCPD and other by-products with high boiling points are withdrawn at the bottom of the first distillation stage and the light substances, such as CPD and small amounts of isoprene, of piperylene and of butadiene, are withdrawn at the top of the second distillation stage and norbornene of high purity is withdrawn at the bottom of the second distillation stage.
Processes for the synthesis of norbornene from DCPD or CPD have been provided in the literature but, in reality, such a synthesis is very difficult to carry out industrially because, due to the low reactivity of ethylene, it requires severe operating conditions. These operating conditions are very close to the conditions for the explosive thermal decomposition of (D)CPD. This is the reason why several industrial plants intended for the synthesis of norbornene have had to cease operations as a result of explosions.
The aim of the present invention is therefore to provide means which make it possible to carry out the industrial synthesis of norbornene under satisfactory safety conditions.
The difficulties of this synthesis will be set out in more detail in what follows.
Pure DCPD is solid at ambient temperature (M.p.=305.15xc2x0 Ki). DCPD is a dimer in equilibrium with its monomer, CPD, via a Diels-Alder reaction. The proportions of the two opposing products depends on the temperature and pressure conditions. The monomerization reaction is endothermic. 
The reaction for the synthesis of norbornene is a Diels-Alder reaction between CPD (the diene) and ethylene (the dienophile). 
This reaction is an equilibrium reaction and theoretically results in a mixture of norbornene (Nor), CPD and DCPD being obtained.
The chemical factors which favourably influence the Diels-Alder reactions are:
a cyclic diene with a high ring strain, which is certainly the case with CPD;
a dienophile activated by an electron-withdrawing group, which is not the case with ethylene.
The reaction conditions have to be adjusted in order to compensate for this lack of reactivity of ethylene. The increase in pressure is a favourable thermodynamic and kinetic factor. The increase in temperature is a favourable kinetic factor but an unfavourable thermodynamic factor. The increase in the ethylene/CPD ratio is a favourable thermodynamic and kinetic factor.
The instability of the products introduces an additional constraint: safety. This is because it is recognized that the synthesis of norbornene is a risky reaction. The low reactivity of ethylene makes it necessary to operate under conditions which are close to the conditions for the explosive thermaldecomposition of the products.
The reaction between ethylene and CPD is strongly exothermic: xcex94Hxc2x0298=xe2x88x9222 kcal/mol ideal gas.
CPD is, in comparison with ethylene, the more sensitive product with regard to these explosive reactions. In 1991, a Union Carbide team (M. Ahmed and M. Lavin, Plant/Operation Progress, 1991, Vol., No. 3, pages 143-154) showed by DSC (differential scanning calorimetry) tests that CPD, heated under pressure, can successively give exothermic reactions at temperatures of:
250xc2x0 C., formation of DCPD oligomers (xcex94H=xe2x88x9266 kcal/mol);
340xc2x0 C., conversion of the oligomers to polymers (xcex94H=xe2x88x9280 kcal/mol);
440xc2x0 C., decomposition of the polymers, resulting in the production of large amounts of gas (xcex94H=xe2x88x9290 kcal/mol).
The above recited temperatures are variable according to the source of the DCPD, without it having been possible to find an explanation for these variations, which can reach 30xc2x0 C. ARC (accelerating rate calorimetry) tests were carried out and made it possible to record the pressure and temperature variations. In these tests, the final decomposition reaction stated at temperatures of 350xc2x0 C. and resulted in increases in pressure of 5 bar (300xc2x0 C.) to 210 bar (460xc2x0 C.).
The reaction conditions are such that other condensation reactions come into play. Thus it is that dimethanooctahydronaphthalene (DMON) is formed. 
Likewise, the formation of tricyclopentadiene (TCPD) is observed. 
These reactions can continue and thus result in the formation of heavy products.
It has now been discovered that stable reaction conditions which make it possible to carry out the synthesis in question on an industrial scale can be obtained if two conditions are combined:
the partial monomerization of the DCPD; and
the production of a fully controlled mixture between the (D)CPD and the ethylene.
Thus, according to the present invention, the synthesis of norbornene is carried out by two successively endothermic (monomerization of the DCPD to CPD) and then exothermic reactions. Surprisingly, and only experience allowed this result to be achieved, it is necessary to have partially monomerized the dicyclopentadiene to cyclopentadiene in order to obtain stable operation of the reaction. Theoretically, the endothermic reaction should be used as much as possible to control the overall exothermicity. The partial monomerization of the DCPD to CPD under pressure makes it possible to obtain a reactant which gives direct initiation of the reaction.
The partial monomerization of the dicyclopentadiene to cyclopentadiene is carried out in an exchanger. It is shown by thermodynamics that, under the pressure (150 bar) and temperature (175xc2x0 C.) conditions, the equilibrium is shifted towards the formation of cyclopentadiene. The temperature was chosen so as to limit fouling of the exchanger and other side reactions. Experience has furthermore shown that, in contrast to intuition, preheating of the dicyclopentadiene is an important point in the intrinsic safety of the plant. The rise in temperature of the DCPD is separated from the reaction region (longer residence time and higher temperature), which limits the risks of drifting towards decomposition reactions.
A subject-matter of the present invention is a process for the manufacture of norbornene from dicyclopentadiene (DCPD) and ethylene, characterized in that the DCPD is subjected to partial monomerization to CPD by preheating:
at a temperature of 140xc2x0 C. to 240xc2x0 C.; and
under a pressure of 20 to 300 bar abs., before reacting it with the ethylene
with an ethylene/DCPD molar ratio of 1 to 20, in particular of 2 to 10;
at a temperature of 200xc2x0 C. to 320xc2x0 C.;
under a pressure of 20 to 300 bar abs.; and
with a residence time of 1 to 10 minutes, in particular of 1.5 to 3 minutes, under stable reaction conditions between the DCPD, the CPD and the ethylene.
The preliminary preheating of the DCPD is conveniently carried out in an exchanger and the reaction proper in a reactor comprising a device which makes possible homogeneous and rapid mixing of the reactants, in order to avoid local overheating, thus eliminating the risks of explosion. The device for the homogeneous and rapid mixing of the reactants is advantageously a device which makes use of the injection of supercritical ethylene in order to disperse the DCPD, preventing the liquid DCPD from ever becoming motionless under the conditions of the reaction. To this end, the reactants arrive in the reactor via the bottom, the DCPD or DCPD-CPD being injected via a first pipe which is positioned axially and which extends over a portion of the height of the reactor and the ethylene being injected via a second coaxial pipe which surrounds the first into the annular region between the two pipes.
In accordance with other characteristics of the process of the present invention:
the ethylene is placed under the temperature and pressure conditions of the reaction region before being introduced into the latter;
the crude norbornene exiting from the reaction region is cooled and subjected to at least one degassing which makes it possible to remove ethylene at the top and to obtain purified norbornene at the bottom; and
the ethylene resulting from the degassing is washed countercurrent wise by the incoming DCPD flow, before being recycled in the incoming ethylene flow.
Another subject-matter of the present invention is a reactor for the manufacture of norbornene from dicyclopentadiene and ethylene, characterized in that it comprises a device for the injection of the reactants arranged in order for the ethylene in the supercritical state to disperse the liquid DCPD or the liquid DCPD-CPD mixture substantially completely under the conditions of the reaction.
In particular, the abovementioned injection device comprises two coaxial pipes for delivery of the reactants via the bottom of the reactor, the said pipes extending axially over a portion of the height of the said reactor, the DCPD or the DCPD-CPD arriving via the central pipe and the ethylene via the annular space between the two pipes, the top part of the system constituting a region of intense mixing of the reactants and the bottom part a region for turbulent flow of the crude norbornene produced, which is discharged via the bottom of the reactor.